【作者】 王志；

【导师】 乔文明；

【作者基本信息】 华东理工大学 ， 化学工艺， 2011， 博士

【摘要】 以NH3作为NOx还原剂的选择性催化还原(SCR)技术是目前应用最为广泛的脱硝技术。然而,该技术中用作还原剂的NH3具有很强的毒性和腐蚀性,给工业操作及存储造成困难；同时,由于废气中NOx浓度的波动会使脱硝过程中NH3的适宜加入量难以精确控制,容易引起由NH3泄漏造成的二次污染。针对以上问题,本文将含NH2化合物担载于催化剂上系统研究了其低温NOx选择性催化还原反应行为,实现了低温范围内高效且高选择性的NOx还原脱除。(1)沥青基球状活性炭(PSAC)担载尿素的NO选择性催化还原反应行为：NO的urea-SCR反应活性显著受到沥青基球状活性炭对NO氧化反应的催化活性的影响。通过计算PSAC上NO的urea-SCR反应和NO氧化反应的表观活化能和反应级数,发现这两个反应具有几乎一致的表观活化能和反应级数,表明NO氧化反应是urea-SCR反应的速率控制步骤。(PSAC上NO的urea-SCR反应的表观活化能为-16.5kJ/mol,NO氧化反应的表观活化能为-15.2kJ/mol;同时,这两个反应均为NO的一级反应,O2的0.5级反应。)进一步的研究表明PSAC上孔径范围在0.5-0.8 nm的微孔催化了NO的氧化反应生成NO2。由NO2不对称分解所产生的吸附态NO3可与PSAC表面担载的尿素迅速反应而被还原为N2。在担载的尿素被完全消耗后,NO2开始释出,且吸附态氮氧化物将逐渐氧化碳表面生成含氧官能团。随后,NO3可以稳定地吸附于被氧化后的碳表面上(2)沥青基球状活性炭担载尿素的NO2选择性催化还原反应行为：NO2的urea-SCR反应不依赖于沥青基球状活性炭微孔的催化作用。提高PSAC上尿素的担载量可以增大NO2与尿素的反应几率,因而有利于NO2的urea-SCR反应活性的提高；同时,尿素担载量的提高延长了NOx的脱除时间。此外,反应进气中NO2和O2浓度的升高均有利于urea-SCR反应活性的提高,但当O2进气浓度大于9 vol%时,继续增加O2进气浓度对urea-SCR反应活性的进一步改善作用变得微弱。(3)炭气凝胶基Mn-Ce复合催化剂担载三聚氰胺的NO选择性催化还原反应行为：在有气相O2存在的情况下,担载在炭气凝胶基Mn-Ce复合催化剂上的三聚氰胺可以高效且高选择性地将NO还原为N2,从而实现较高的NOx转化率。催化剂上担载的三聚氰胺在SCR反应过程中被完全消耗,没有含N副产物以气体形式释放或在催化剂表面残留。提高炭气凝胶基Mn-Ce复合催化剂上三聚氰胺的担载量可以延长NOx的脱除时间。但高于15wt.%的三聚氰胺担载量会造成催化剂上金属氧化物活性位被严重覆盖,因而有害于NO的melamine-SCR反应活性。高于400℃的催化剂煅烧温度会减弱催化剂上Mn与Ce的氧化物的相互作用,且促使金属氧化物形成更大的晶体颗粒和更加规整的晶型结构,这些结果均有害于NO的melamine-SCR反应活性。反应温度的升高和O2进气浓度的增加均有利于NO在炭气凝胶基Mn-Ce复合催化剂表面的化学吸附生成可与三聚氰胺反应的吸附态N03-,因而有利于催化剂上melamine-SCR反应的进行。(4)炭气凝胶基Mn-Ce复合催化剂担载三聚氰胺的NO2选择性催化还原反应行为：NO2与三聚氰胺的反应依赖于炭气凝胶基Mn-Ce复合催化剂的催化作用。高于15wt.%的三聚氰胺担载量和高于400℃的催化剂煅烧温度都有害于NO2的melamine-SCR反应活性。反应温度的升高和O2进气浓度的增加由于都强化了NO2在炭气凝胶基Mn-Ce复合催化剂表面的化学吸附生成可与三聚氰胺反应的吸附态N03-,从而有利于催化剂上melamine-SCR反应的进行。(5)低温下melamine-SCR反应的机理研究：通过选取合适的反应温度,SBA-15基Mn-Ce复合催化剂同样可以有效地催化(?)nelamine-SCR反应。SBA-15基Mn-Ce复合催化剂上NOx的吸附和melamine-SCR反应的原位红外研究表明：(1)在没有气相02存在的情况下,NO和NO2均可在SBA-15基Mn-Ce复合催化剂表面化学吸附生成吸附态N03-。气相O2的加入有利于NO在催化剂表面的化学吸附,而对N02的化学吸附影响不大。(2)低温下SBA-15基Mn-Ce复合催化剂上的melamine-SCR反应较为复杂,由多个基元反应组合而成。在反应初期,参与吸附态NOx还原的官能团主要是三聚氰胺分子中的NH2,反应产生N2和H2O；当反应进行到一定程度,由反应中间产物水解和分裂而成的吸附态HNCO开始参与吸附态NOx的还原,生成C02、N2和H2O。更多还原

【Abstract】 Selective catalytic reduction (SCR) with NH3 is the most widely used method for the removal of NOx. However, NH3 does not appear to be an ideal reducing agent when considering its corrosiveness and toxicity. Furthermore, it is very difficult to exactly control an appropriate NH3 input because of the fluctuating NOx concentration in exhaust gas, which is very likely to cause additional environmental problems due to NH3 slip. Therefore, it is of great significance to develop new SCR technologies with other proper reducing agents as a substitution for NH3. In this work, the low-temperature selective catalytic reduction of NOx with NH2-containing compounds supported on catalysts was systematically studied. The details are shown as follows:(1) Low-temperature SCR of NO with urea supported on pitch-based spherical activated carbon (PSAC):NO oxidation to NO2 catalyzed by the 0.5-0.8 nm micropores in PSACs was found to be the rate-limiting step in urea-SCR reaction, which was confirmed by both the apparent activation energy calculations and the kinetics results of urea-SCR reaction and NO oxidation on PSAC. These two reactions gave very similar negative apparent activation energies (-16.5 kJ/mol for urea-SCR reaction and-15.2 kJ/mol for NO oxidation), indicating that the adsorption of reactants on PSAC is of key importance in these two reactions. Moreover, these two reactions were both approximately first order with respect to NO and one-half order with respect to O2. It was found that NO3 from the disproportionation of the produced NO2 was quickly reduced by supported urea into N2. After the complete consumption of supported urea, NO2 started to release, and the carbon surface was gradually oxidized by adsorbed NOx species. NO3 was found to be stably adsorbed on the oxidized carbon surface.(2) Low-temperature SCR of NO2 with urea supported on PSAC:The urea-SCR of NO2 was not catalyzed by the micropores in PSACs. Increasing urea loading raised the reaction probabilities of SCR of NO2 by urea, which resulted in significant increase of the SCR activity; moreover, the NOx removal period was extended. It was found that the SCR activity was improved by increasing NO2 or O2 concentration in the feed gas. However, further increase in O2 concentration above 9 vol% made a weak contribution to the improvement of the SCR activity.(3) Low-temperature SCR of NO with melamine supported on carbon aerogels-supported MnOx-CeO2 based catalyst:In the presence of gaseous O2, carbon aerogels-supported MnOx-CeO2 based catalyst with 15 wt.% melamine loading exhibited high activity and selectivity in the SCR of NO to N2. It was found that, after the SCR reaction, melamine supported on the catalyst was totally consumed, and no N-containing by-products released as gas or deposited on catalyst surface during the reaction. Increasing melamine loading extended the NOx removal period. However, melamine loading above 15 wt.% markedly decreased the SCR activity due to serious coverage of active sites. The calcination temperature above 400℃caused the decrease of the interactions between the Mn and Ce oxides and increased the crystallinity of the metal oxides. Such changes on the catalyst were found to be harmful to the melamine-SCR activity. Since the increase of O2 feed concentration and reaction temperature both strengthened the chemical adsorption of NO on catalyst surface to form adsorbed NO3- which was reduced to N2 by supported melamine, the melamine-SCR activity increased.(4) Low-temperature SCR of NO2 with melamine supported on carbon aerogels-supported MnOx-CeeO2 based catalyst:The melamine-SCR of NO2 was catalyzed by carbon aerogels-supported MnOx-CeO2 based catalyst. Melamine loading above 15 wt.% and calcination temperature above 400℃were both found to be harmful to the SCR activity. Since the increase of O2 feed concentration and reaction temperature both strengthened the chemical adsorption of NO2 on catalyst surface to form adsorbed NO3- which was reduced to N2 by supported melamine, the melamine-SCR activity increased.(5) Mechanism study on low-temperature melamine-SCR:SBA-15-supported MnOx-CeO2 based catalyst was also found to efficiently catalyze the SCR of NOx with supported melamine at proper reaction temperatures. In this work, the systematic in situ FTIR studies on NOx adsorption and melamine-SCR reaction over SBA-15-supported MnOx-CeO2 based catalyst showed that:(1) In the absence of gaseous O2, both NO and NO2 could be chemically adsorbed on catalyst surface to produce adsorbed NO3-. The introduction of O2 in the feed gas benefited the chemical adsorption of NO while had no obvious effect on the chemical adsorption of NO2. (2) The low-temperature melamine-SCR reaction which contains a series of elementary reactions is very complex. At the early stage of the reaction, the functional groups involving in the reduction of adsorbed NO3- were mainly the NH2 groups in melamine molecules, the products contained N2 and H2O. When the NH2 groups were consumed to some extent, the adsorbed HNCO from the hydrolysis and decomposition of reaction intermediates started to participate in the reduction of adsorbed NO3- to produce CO2, N2 and H2O.更多还原